What is a megaprim and pros and cons

As a newbie i need to understand what is a megaprim and the advantages and disasvantages.

Thanks in advance

Well the advantages is that you can go over then 10x10x10 limit SL has put up. I used it to build a part of my store with (WIP still). It comes i think in a pack of 10 or 20 prims in different sizes (i need to look it up which ones)

The disadvantages: yes, the biggest that bugs me the most is that you CANT resize it to your desired size. It jumps back to 10 x 10 x 10, which makes me scream in agony from time to time. Also the fact that you cant copy it decently in SL with the shift button, but have to drag it from out of the inventory again makes me sometimes scream, but hey, everything has an advantage and a disadvantage